The subject matter disclosed herein relates to a Ram Air Turbine (RAT) and, more particularly, a motor assisted fine pitch startup RAT.
The latest generations of RATs are designed to operate with an initial blade pitch, which is in the range of 5 to 15 degrees with respect to an axis perpendicular to the flow direction of incoming air. This initial blade pitch angle is chosen to maximize performance of the RAT during landing when the demands on the RAT are at high levels but the available energy in the airstream is low.
FIG. 1 shows the start up and steady state performance comparison between setting the initial blade pitch at a small blade pitch angle and at a large blade pitch angle. As shown in FIG. 1, using a small blade pitch angle, such as 9 degrees, for the initial blade pitch fails to provide an optimum amount of torque for allowing the RAT to start from 0 rpm under low airspeed conditions since the torque produced by the RAT at that blade pitch is lower than the load torque. That is, the small fine pitch blade angle has less torque at low speeds than a relatively larger blade angle and this low torque results in small torque margins compared to the turbine resisting torque and the result is relatively long start times for the RAT. These long start times create controllability issues for the aircraft that must be countered with larger batteries and or hydraulic accumulators.
At steady state operating speeds, for example at approximately 5000 RPM in FIG. 1, the smaller blade angle provides more torque than the larger blade angle. Consequently in FIG. 1, the 9 degree angle would be preferred over the 15 degree angle at steady state operating speeds.
Given the low speed versus operating speed characteristics of a fine pitch startup RAT, RATs designed with a low initial blade angle (“fine pitch start RAT”) typically must trade off steady state performance versus startup performance. This trade off results in larger turbines, lower performance and/or slower start times.
This issue has been addressed by the use of mechanisms that allow the RAT to start at a larger blade angle (coarse pitch start RAT) with a transition back to a fine angle at an intermediate turbine speed. After this transition occurs, the RAT reverts to normal operation like any other fine pitch RAT. However, these mechanisms allowing the RAT to start in coarse pitch and then transition to fine pitch increase the weight and size of the turbine while decreasing reliability.